Certain characteristics of signals in electronic circuits may drift from time to time as a result of undesirable circuit characteristics, or interference from other signals. Once identified, compensation may be added to a circuit to cancel out drift, or the circuit may be redesigned to eliminate the reason or source for the drift.
The problem of signal and ground drift in certain control systems is particularly serious where signal strengths are quite low, and the sources of signal drift are rather strong in comparison. For example, in temperature control systems, a sensed temperature signal of relatively low signal strength is fed back to a temperature control circuit that must be periodically calibrated to assure its accuracy. As a means of calibrating circuit, a calibration signal is applied to the control circuit which also receives the temperature signal. Since the control circuit employs a common input employed with a common ground, a certain amount of residual bias or offset current is present in this input circuit which results from the last signal that was input to the temperature control circuit. This residual bias or offset is either subtracted from or added to the applied temperature or calibration signal, thus creating a certain amount of signal error. The input signal is later compared to threshold values and, because of the signal error, the threshold comparison my yield an incorrect result, e.g., the signaling of an alarm when in fact the input signal does not truly indicate an alarm.
Thus, there is a clear need in the art for an improved circuit arrangement, particularly for temperature control systems, which eliminates bias, offset or drift of input signals.